1. Field of the Invention
The present invention relates to a method for removing chemical oxygen demand (COD) from wastewater by electrolysis and oxidization. More particularly, it relates to a method utilizing a fluidized bed to facilitate the electrolytic reduction of ferric ions so as to sustain a high COD removal efficiency. The method described herein is particularly suitable for purification of highly concentrated wastewater.
2. Description of the Related Arts
To meet strict laws on environmental protection, the COD in effluent wastewater from factories must be reduced to a significant extent. A feasible method, known as Fenton's method, has been widely used for the reduction of COD in wastewater. According to Fenton's method, hydrogen peroxide and iron(II) compounds are added to wastewater so that the organic pollutants contained in wastewater are oxidized by the hydroxyl free radicals (OH) produced by the reaction of the hydrogen peroxide and the ferrous ion. However, in practical applications, Fenton's method is not completely satisfactory. The disadvantages thereof are summarized as below:
1. The necessity of addition of chemical reagents such as hydrogen peroxide, ferrous ions, acids and alkalis makes the practice of this method costly. PA1 2. A significant amount of iron(III) hydroxide sludge is produced. The iron sludge needs to be further treated. However, such treatment is also costly and may further pollute the environment.
Several modifications of Fenton's method have been proposed to solve the above-mentioned problems. For example, Gregor et al (EP 92/02357) discloses a process to purify highly concentrated wastewater by chemical oxidation using Fenton's reagent (i.e. hydrogen peroxide and iron(II) compounds). The process is characterized in that the iron(III) sludge is separated and then reduced electrolytically back into iron(II) compounds and in that the sludge so reduced is fed back into the process. Japanese Patent JP 62171732 discloses a method for treating organic matters in waste cleansing systems by Fenton's method, wherein the iron(III) is electrolytically reduced to iron(II) and then incorporated into the iron sludge for reuse.
The method as described in the last paragraph make the reuse of the iron sludge possible. However, such a method has a serious drawback, namely, a gradual decrease in reduction efficiency. In the electrolytic reduction, the concentration of iron(III) must be greater than 30,000 mg/L so as to reach a high efficiency and to provide sufficient iron(II) to carry out a Fenton's reaction. However, since the current efficiency at a cathode gradually decreases with the consumption of iron(III), the COD removal efficiency as well as the reduction efficiency gradually decreases with time. In Gregor's modification, when two-thirds of the iron(III) is reduced to iron(II), the electrolytic reduction efficiency decreases to about 50%. Accordingly, it would be time-consuming and uneconomic to completely removal the COD in wastewater because the reaction time will be undesirably prolonged. According to Gregor's method, the electrolytic reduction is carried out at a pH of less than 1; while on the contrary, the oxidization is carried out at a pH greater than 3. This suggests a large quantity of acids and alkalis would be required to adjust the pH values.
A diagram of the current efficiency and the Fe(II) concentration as a function of the reaction time is shown in FIG. 1, in which a variety of cathodes are employed at the electrolysis. It points out in FIG. 1 that if the initial concentration of Fe(II) is 10,000 mg/L, it will take longer than 150 minutes to reduce two-thirds of the Fe(III) to Fe(II), whereas the current efficiency has diminished to less than 20%.